Reproducible Sounding Rocket Payload Design and Implementation

University of Colorado at Boulder

Authors:

Nicholas Ethan Bradley

Philip Zanin Holtzman

Lee Eduardo Zetterstrom Jasper

Emily Caroline Walters

Faculty Advisor:

Chris Koehler, Colorado Space Grant Consortium

1.0Abstract

To provide affordable sounding rocket payloads to students and university groups, a general model for design, production, and implementation of a scientifically integrated system is discussed. Describing the project template in full involves discussing the necessary subsystems, possible science experimentation, and design issues and requirements involved in developing such a program. The RocketSat model developed at the University of Colorado demonstrates knowledge necessary to begin and finish a successful sounding rocket payload program. This general model was used in a September 2006 launch with inconclusive results due to the failure of the launch vehicle. An expanded payload will be flown in the spring of 2007, and should return valuable scientific data and a successful checkout of the integrated payload model. Merits and drawbacks are noted. This design significantly reduces cost compared to more conventional rocket payloads, and still provides ample leeway for beneficial and educational experimentation, particularly in a university research environment.

2.0System Concept, Requirements, and Goals

It may seem that student sounding rocket payloads are merely another option for hands-on research that a collegiate engineer possesses. Though it is true that the design and construction experience gained from such a program can be just one type in a vast sea of opportunities, rocket payloads are unique in the regard that they provide students with the ability to fully manage and participate in a project from concept to resolution. Students are charged with the task of conceiving and constructing a scientifically valid payload with a complete power, structural and electronic interface system that will produce actual numerical data up to more than 100 km in altitude. Because of the recent advent of affordable space access to commercial and educational groups, these types of programs are feasible for a university research environment. This leads to the necessity of the development of a standardized, reproducible interface that can be used for a multitude of different needs and requirements. The RocketSat program at the Colorado Space Grant Consortium at the University of Colorado uses such a system, and has been able to successfully develop its first flight generation for use in a commercial space launch.

The concept of the RocketSat program began with the seemingly simple task of populating a 7.8 inch diameter aluminum disk with scientific hardware. The initial goal of the program was to eventually expand the functionality of the single plate to a system of several stacked plates, interfaced as necessary. Thus, the RocketSat Integration and Structural Equipment Restraint (RISER) system was created. Beyond the external requirements set by the launch provider (mass, size, etc.), RocketSat’s RISER had several driving internal requirements:

  • The system shall use commercial-off-the-shelf (COTS) parts
  • The individual levels contained within the RISER shall be independent of each other to allow a variety of plates to be matched to the system
  • The system shall return valid, reproducible scientific data from high-altitude flights

These goals provided a framework around which to develop the RISER system and the RocketSat program. A demonstration of a fully-integrated RISER system is visible in Figure 1. Specific goals for the RocketSat II integrated RISER system included scientific measurement systems such as a Geiger counter, a microwave radiation detector, accelerometers, temperature sensors and pressure sensors. In addition, the structural restraint system had various requirements for flight to provide a full proof-of-concept for the design. RocketSat II has not yet been launched, but it carries high expectations for success. The RISER system has proven to be an efficient, well-designed model, and the upcoming proof-of-concept flight will demonstrate its robustness and reliability.

As with any research and design project, there are invariably pitfalls to be aware of, as well as suggestions of design concepts that have proven to be beneficial to the project as a whole. Given that the RISER system was designed as an undergraduate engineering experience, the subsystems cannot contain a level of complexity that is either too difficult for a small group to design and implement, or difficult to reproduce per the driving RocketSat overall system requirements. For these reasons, it is highly advisable to use COTS parts for a RocketSat/RISER payload. Due to the space limitations on each plate in the system, the simplicity required to develop a portion or portions of the payload necessitates only the required subsystems: power, science, structures, and Command and Data Handling (CDH). To maintain a viable project for a workshop-type environment, it is important to use components that have good documentation, are in production, and have a reliable support base.

Using RocketSat II as a specific example of a payload developed using the RISER system, the three main subsystems are detailed according to the design, implementation, and construction processes. The following sections contain an overview of the structural, scientific, and CDH systems for the RocketSat II payload, which is due to launch in late April of 2007. RocketSat II has proven to be a successful system of interchangeable plates to provide quality scientific data obtained in Earth’s upper atmosphere, and should be taken as an example.

3.0Science

Design and Requirements

The goal of the RocketSat program is to develop a platform for scientific experiments that take advantage of the high altitude space environment. The design process for this flight additionally involves the study of the dynamics of the rocket flight in order to provide information for future payloads about the flight conditions. For this flight, ten experiments were chosen as described below. These experiments are driven by the overall goal to provide a platform for future workshops. Most of the experiments are completely reproducible for a class type design and provide ideas of sample experiments. Several of the experiments are either individual parts with ample instructions on integration and use, or kits that allow for most students with limited payload design knowledge to implement with relative ease. At the same time, however, the Global Positioning System(GPS) experiment as described below involves the more unique research of a local professor and therefore is not a simple experiment for any group but rather tests the possibility of larger experiments and provides a platform for faculty research.

- Magnetometer - The magnetometer shall record data of the change in the Earth’s magnetic field as a function of the altitude. Also, it shall detect any large changes in electric fields as affected by the rocket flight.

- Geiger counter – The Geiger counter shall detect radioactive particles (primarily gamma) that contact it in order to determine the amount of radiation as a function of altitude. This experiment utilizes a kit to build the Geiger counter, which provides a simple experiment for use in a class.

- GPS – This is by far the largest experiment for the flight and is done with the support of a local professor’s research. The GPS detects the raw data signal from the GPS satellites in view and records them to memory using a small onboard computer. The data, during post processing, will be interpreted and the effects of the dynamic rocket flight as well as the high altitude effects on the GPS will be measured. This experiment does not follow the idea of an easily reproducible payload directly since it weighs just over two pounds and requires multiple levels in the RISER system.

- Accelerometers – The goal of the accelerometers (x, y and z axis) is to develop a dynamic rocket flight profile. The accelerometers will sense the overall forces and vibrations that affect the RISER levels as well as the rocket as a whole.

- Microwave sensor – The microwave sensor shall detect the microwave radiation with increased altitude to develop an altitude profile of such radiation density.

- Temperature – This sensor shall detect changes in the temperature as a function of altitude. Similar to the accelerometers, little data is available of the temperature that the payload will experience. This data will be ideal for future flights in order to design future payloads that can function ideally at the flight temperatures.

- Pressure – The pressure sensor shall measure the ambient pressure of the air surrounding the internal payload. Since the rocket is not sealed, theoretically, most of the air should escape into space as the air pressure deceases with increased altitude. The pressure sensor shall detect this change.

- Humidity – The humidity sensor shall measure the amount of moisture in the inside of the payload throughout the rocket flight.

- Strain gages – The strain gages shall detect the strain that the structure experiences during the flight. Two rosette gages (3 sensors at different angles) are on different parts of a plate to measure any plate deflections during flight. Similarly, one linear sensor is on a vertical standoff to detect any strain imparted upon it during flight.

- Camera – The camera shall record the view from the payload window and record the acoustic environment throughout the flight. This involves modifying the camera in order to turn it on and record through CDH command.

Testing

In order to verify the science payloads, several tests leading up to a full system test were performed. Initially, this began with testing the components before any integration depending on the experiment and need for additional components. For example, the magnetometer was first tested by holding a small magnet at different distances and reading the magnetometer output with a multimeter. However, some experiments, such as the strain gages, could not be tested without additional circuitry to read the sensors.

The next level of testing involved integrated testing. This proved to be the most complicated and time consuming, as with most any design project. Each sensor was connected electronically and tested with the CDH system to both turn on and collect appropriate data. After verifying that the science system and CDH were functioning correctly, the structure was integrated. This allowed for full system testing. Not only is this to check system functionality, but also, it provides a baseline reading for the instruments. The most complex challenge for full system testing is the interference of the different aspects of the system. Once full system testing has been completed with all components and their interactions function properly, the payload can be deemed ready for flight.

4.0Command and Data Handling and Power

The Command and Data Handling subsystem, like the other subsystems of the RocketSat project, is designed with simplicity and reproducibility as a top priority. The ultimate goal of the system is to be a generic infrastructure which can be built upon for future payloads.

Power System

The RocketSat payload has four separate power systems. The camera has an internal 3V battery and GPS experiment has its own large battery due to the power needs of the GPS computer. The remaining systems are powered by Lithium-Ion 9V batteries. These have a lifetime of roughly 900 mA-hours and are rated for the conditions of rocket flight. 100 mA is the maximum recommended draw for a single 9V; therefore, using two batteries in parallel will fulfill the power needs with a large enough factor of safety to allow for a few hours of launch slipping without incident.

The power system is controlled using a G-switch and solid-state relay configuration. This ensures that power begins to flow immediately upon launch when roughly 7-G forces are detected. The solid-state relay then “latches out” the G-switch ensuring that it cannot trigger again and interrupt power. This solution has proven very effective through testing and is easily constructed and implemented. If pre-launch power is required, as is the case with parts of RocketSat II, the power can be controlled with both a G-switch and a flight switch. While a more complex solution, this can also be implemented by a student with a minimal amount of electrical experience.

Command Hardware and Software

The command hardware used on RocketSat has progressed over the generations. For RocketSat I, the payload was controlled by two Acroname BrainStems. These controllers, chosen for their simplicity in programming, are not ideal for the needs of the RocketSat concept. In particular, the BrainStems have a low sampling rate which limits the speed with which data can be sampled and a high current power draw.

The second generation of RocketSat uses one Atmel Atmega AVR microcontroller for each of the two payload systems. These microcontrollers have a much higher sampling rate, use very little power, and are programmed using the C programming language. Priced at just eight dollars each and with a large amount of support libraries in place, the AVR is both an affordable and effective microprocessor for a sounding rocket payload. The AVR has the ability to communicate using Inter-Integrated Circuit (IIC) protocol, Serial Peripheral Interface (SPI) protocol, and Universal Asynchronous Receive and Transmit (UART) protocol and has two external interrupts that can be utilized.

The memory system of the RocketSat payload is built with electronically erasable programmable read-only memory (EEPROM). RocketSat employs both 512kb and 1024kb EEPROMs for data storage. EEPROMs have many benefits including a small package size, easy communication using the IIC standard, the ability to use several EEPROMs in series for greater memory capacity, and the generosity of suppliers to give free samples. While other forms of memory such as SD cards or USB flash memory can be utilized when more capacity is needed, these introduce a higher level of complexity into the project. USB memory is used with the GPS experiment because of the extremely large quantity of data sampled.

Controlling Experiments

Another purpose of the AVR is to control experiments and give commands. Both the camera and GPS, while self-sufficient once operating, require signals from the AVR to turn on power and/or begin recording. This is accomplished using Field Effect Transistors (FET) to essentially replace a power switch or record button. The FET, when given a high voltage on the gate, creates a high voltage across the power pins on the GPS computer, starting the boot procedure. Likewise, a high voltage across the power or record leads on the camera initiate power-up or the record-function, respectively.

Analog vs. Digital Signals

One of the most important roles of the CDH system is to fulfill the needs of the numerous science experiments. For each experiment, a decision was made whether to pass the data as an analog signal or a digital signal. In many cases, this decision was predetermined by the experiment itself. The accelerometers, for example, output a varying voltage corresponding to acceleration. A continuously changing voltage such as this one is a natural candidate for an analog signal. The Geiger counter, on the other hand, outputs a voltage pulse whenever a radiation particle is detected, lending itself to being handled as a digital signal. The Geiger counter utilizes one of the AVR’s external interrupts to record the number of radiation hits detected.

Furthermore, the AVR only has eight input/output (I/O) pins which are capable of reading an analog signal, therefore limiting the amount of analog signals that can be processed. This problem can be circumvented, however, by using an analog to digital converter (ADC). An ADC gathers into bins, much like a histogram, the analog signal so it can be read digitally. The analog pins on the AVR employ an internal ADC which accomplishes the conversion task in the same way. By using a high precision ADC, an analog signal can be read by a digital I/O pin without any appreciable loss in precision. For the RocketSat project, an ADC is used with the microwave sensor experiment to read its analog output into a digital I/O pin.

A 24-bit ADC is also used with the strain gage experiments due to both the lack of available analog I/O pins and the experiment’s requirements for high precision. Since there are seven strain gages, an 8:1 multiplexer (MUX) is used so that all seven strain gages can be input to one digital I/O pin. The MUX cycles through each of the seven inputs creating one data stream containing all of the strain gage data. Theoretically, an AVR could collect data from a near limitless number of experiments by using MUXs and ADCs. The caveats in this situation are the lower sampling rate for any one experiment and the increased soldering difficulty of numerous surface-mount components.